In the manufacture of the highest performance devices today, two dissimilar semiconductors must be placed in close proximity to each other; as a matter of fact, an abrupt interface is necessary. For example, In.sub.x Ga.sub.1-x As on GaAs is a technologically important system since high speed devices such as high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs) may be manufactured from this material. The more In that is added to the In.sub.x Ga.sub.1-x As, the better the properties. The properties of the In.sub.x Ga.sub.1-x As can be estimated from linearly extrapolating between the binary compounds InAs and GaAs.
The problem that occurs in the majority of the cases is that two dissimilar semiconductors that have dissimilar electronic properties (the advantage) have dissimilar structural properties as well (the disadvantage). For example, as one adds more In to In.sub.x Ga.sub.1-x As to take advantage of the electronic properties of the In.sub.x Ga.sub.1-x As and the In.sub.x Ga.sub.1-x As/GaAs abrupt interface, one also increases the size of the lattice constant. Therefore, when one deposits crystalline In.sub.x Ga.sub.1-x As on GaAs, defects form at the interface due to the difference in the size of the crystal structure. These defects can destroy the electronic properties of the material since they can trap electrons and holes. This property of the crystalline defects can be used to actually image the defects, identify them, and determine when they have been eliminated. Thus, it would be beneficial to fabricate mismatched semiconductor materials having an abrupt interface and reduced or eliminated defect density.
The lattice-mismatched semiconductor systems have been extensively investigated for the realization of strained-layer devices. Promising devices have been fabricated by Rosenberg, J. J. et al, IEEE EDL 6:491 (1985); Ramberg, L. P. et al, J. Appl. Phys., 61:1234 (1987); and Katsumoto, S. et al, J. Appl. Phys., 24:636 (1985) using the InGaAs/GaAs system. However, higher In concentrations and thicker overlayers are needed in the future for increased device performance In order to realize these devices, the dislocation density created by the lattice mismatch for high In or large epilayer thicknesses must be reduced, or, preferably eliminated. Therefore, the nucleation sites for misfit dislocations must be prevented from operating or their effect must be minimized Nucleation mechanisms such as threading dislocations, multiplication of misfit dislocations, and surface dislocation half-loop formation have been discussed by Matthews, J. W. et al, J. Appl. Phys., 41:3800 (1970); Thin Solid Films. 33:253 (1976); and J. Vac. Sci. Technol., 12:126 (1975). Dislocation multiplication has been discussed in detail by Hagen and Strunk, Appl. Phys., 17:85 (1978). Matthews originally attributed misfit dislocation formation to the bending over of threading dislocations. It was proposed that if threading dislocations were the source of misfit dislocations, then by limiting the lateral dimension of the sample before growth, not enough threading dislocations would be present to nucleate misfit dislocations and therefore the interface defect density would decrease. However, it was observed that density of misfit dislocations at the interface is greater than the number of threading dislocations. Hagen and Strunk proposed the interaction of misfit dislocations as an additional source of misfit dislocations, and Matthews considered the surface nucleation of dislocation half-loops.